The object of the invention is a filter structure formed by resonators comprising a conductive housing, so that the filter structure is suitable to be used generally on microwave frequencies, for instance as a duplex filter in base stations of mobile communication networks, in satellite links and in WLL (wireless local loop) networks.
When the requirements placed on the filter are relatively stringent, for instance regarding the transition band between the pass band and the stop band and regarding the attenuation on the stop band, then the order of the filter will become relatively high. In the case of a resonator filter this means that the structure consists of a number of single resonators and couplings between them. The structure is relatively complicated and its manufacture causes substantial costs. Thus the essential question is how low manufacturing costs we can obtain with a filter which meets the requirements.
A great number of filter structures based on resonators are previously known. Usually the resonators are arranged in a row, so that outwardly they form a uniform metal housing. The most common resonator type is the coaxial quarter-wave resonator. The coupling between the resonators is arranged either capacitively or inductively with the aid of auxiliary components. In its details the coupling mechanism may vary quite considerably. FIG. 1 shows an example of such a prior art filter, partly opened and disassembled. It contains in a row four coaxial resonators 110, 120, 130 and 140. Each resonator comprises an inner conductor, such as 131, and an extension part for it, like 132. The extension part increases the capacitance at the upper end of the structure, in other words at the open end, whereby the resonator can be made shorter in the vertical direction. Each resonator further comprises an outer conductor which is formed by the resonator partitions, by side wall parts of the whole resonator housing, and for the outermost resonators by the gable walls. The structure operates as a quarter-wave resonator because each inner conductor is at its lower end connected to the filter""s conducting bottom plate 105 acting as a part of the signal ground. Thus the line formed by the inner and outer conductors is short-circuited at its lower end. The structure is covered by a conductive lid, so that the filter housing is closed. It must be observed that a resonator in this description and in the claims means logically the whole resonating structure, not only the inner conductor. FIG. 1 shows as an example one capacitive coupling and one inductive coupling between the resonators. The capacitive coupling is between the resonators 110 and 120 at their open ends where the electrical field is relatively strong. The partition 106 between the resonators 110 and 120 has an opening 107 for the capacitive coupling. At the ends of the inner conductors of said resonators there are fastened blades 114, 123 which are directed towards this opening, thus increasing the coupling capacitance between the resonators. Via the connector 101 the filter is fed also capacitively with the aid of the blades 103, 113. The inductive coupling is between the resonators 120 and 130, close to their shorted ends where the magnetic field is relatively strong. For the inductive coupling a body 125 of conducting board is shaped so that it extends close to the inner conductor of said resonators and that it is grounded in suitable places. The body 125 generates a mutual inductance between the resonators. The feed of the resonators could also be realised inductively. A disadvantage of the described structure and of corresponding structures is that it is cumbersome to tune the filter, which entails costs. Also the actual manufacture before the tuning generates relatively high costs.
FIGS. 2a and 2b shows another example of a prior art structure (Fl 970525). The basic structure of the filter is similar to that of FIG. 1. In the FIG. 2a there is seen of the structure the bottom plate 205 and three central conductors 221, 231, 341. Close to the short-circuited ends of the resonators there is a dielectric board 210 parallel with the bottom plate 205. The inner conductors extend through the dielectric board via the openings seen in it. The coupling between the resonators is now provided by conductor strips formed on the dielectric board 210. FIG. 2a shows two such conductor strips 211 and 212. FIG. 2b is a top view of the board 210. In this example the conductor strip 211 forms a loop around the central conductor 221, and an incomplete loop around the central conductor 231. Nearest to the central conductor 231 there is a separate conductor loop 212, which shows a mutual inductance with the conductor strip 211. Alternatively, the dielectric board can be also at the bottom of the structure, so that its outer coating replaces the bottom plate 205. Such structures are advantageous to manufacture. However, a disadvantage is the dielectric board having an effect which increase losses and reduces the compactness of the structure.
The objective of the invention is to present a new way to realise a resonator filter which reduces the disadvantages. The filter structure according to the invention is characterised in that what is presented in the independent claim. Some advantageous embodiments of the invention are presented in the dependent claims.
The basic idea of the invention is as follows: in the manufacturing phase of the filter housing that side of the housing left open is closed with a conducting lid. For the couplings between the resonators a dielectric board is placed outside the conducting filter housing, over some wall and at a suitable distance from it. The couplings are realised with the aid of conductor areas made on the dielectric board and openings made at places corresponding to them in the filter housing. The coupling energy is conveyed through the opening from the resonator to the field of one transmission line on the dielectric board, and from there through another opening to another resonator. In order to make a stronger coupling it is possible to fasten to the conductor area on the dielectric board a coupling element, for instance a thread-like or plate-like conductor which extends through an opening into the housing close to the inner conductor of the resonator. Further it is possible to make openings in the resonator partitions in order to obtain a desired coupling. Also the transferring of signals to the filter and from the filter can be realised utilising said dielectric board as a support structure. The main part of the filter housing and the resonator""s inner conductors are manufactured as an integral part using extrusion, casting, tooling or some joining technique.
An advantage of the invention is that the manufacturing costs of the filter are relatively low, because the number of separately mounted components is relatively low. A further advantage of the invention is that the tuning costs of the filter are relatively low, because the printed board couplings are fixed, whereby only relatively simple fine tuning is required. A further advantage of the invention is that the filter has relatively stable characteristics due to the uniform resonator structure. A further advantage of the invention is that the dielectric board used for the couplings does not introduce any substantial losses, because it is outside the filter housing. Due to the same reason it can be made of a cheaper material than in the prior art solutions where the printed board is within the resonator structure. Furthermore, an advantage of the invention is that it enables advantageous methods for the temperature compensation of the filter. The methods are based on the choice of the printed board laminate and on the addition of appropriate conductor areas and/or elements to the printed board. A further advantage of the invention is that it is relatively simple to modify the filter. A further advantage of the invention is that on the filter""s printed board it is possible to integrate amplifiers, directional couplers, dividers, adders and antennas or parts of antenna structure which are closely related to the filter.